1. Technical Field
The present invention relates to a surface light source device and a liquid crystal display device, and specifically to a surface light source device that is used as a backlight for a liquid crystal display, and a liquid crystal display device in which the surface light source device is used.
2. Related Art
Nowadays, there is an increasing demand for a low profile of a surface light source device with the progress of a low-profile mobile device provided with the surface light source device. In order to reduce a thickness of the surface light source device, it is necessary to reduce a thickness of a light guide plate. However, even if the flat light guide plate can be reduced in thickness, there is a limitation to reducing a height of a light source including an LED. Therefore, in the case where the thin, flat light guide plate is used, the height of the light source is larger than a thickness of an end surface (a light incident surface) of the light guide plate, and the light source disposed opposite to the light incident surface of the light guide plate projects above from an upper surface of the light guide plate. When the light source projects above from the light guide plate, light emitted from the light source is not entirely incident to the light incident surface of the light guide plate, and the light partially leaks to the outside to degrade light use efficiency.
In order to solve the above problem, there has been proposed a use of a light guide plate, in which a light introduction part having a larger thickness than a thickness of a flat light guide plate body is provided at an end of the light guide plate body and an inclined surface inclined from a maximum thickness point of the light introduction part toward the end of the light guide plate body is provided in the light introduction part. For example, WO 2010/070821 and WO 2008/153024 disclose the surface light source devices in each of which the light guide plate is used.
FIG. 1 illustrates an example of a surface light source device 11 in which the light guide plate including the light introduction part thicker than the light guide plate body is used. A light guide plate 13 includes a light guide plate body 14 having a substantially even thickness and a wedge-shaped light introduction part 15. A deflection pattern or a diffusion pattern is formed in a rear surface of the light guide plate body 14, and a lenticular lens 16 is formed on a surface thereof. An inclined surface 17 is formed in the light introduction part 15. The inclined surface 17 is inclined from a point having the maximum thickness of the light introduction part 15 toward an end of the light guide plate body 14. The thickness of an end surface (a light incident surface) of the light introduction part 15 is larger than a height of the light source 12. In the surface light source device 11 in which the light guide plate 13 is used, the thickness of the end surface of the light introduction part 15 is larger than the height of the light source 12, whereby the light introduction part 15 efficiently takes in the light emitted from the light source 12. The light taken in by the light introduction part 15 is guided to the light guide plate body 14 and spread in a flat manner, and the light is reflected by the deflection pattern or the diffusion pattern, and output to the outside from a light exit surface of the light guide plate body 14. At this time, a directional pattern of the light output from the light exit surface is spread by the lenticular lens 16. Therefore, in the surface light source device having the above structure, not only the light use efficiency of the light source can be improved but also the low profile of the surface light source device can be achieved.
However, for the surface light source device 11 in which the inclined surface 17 is provided in the light introduction part 15 as illustrated in FIG. 1, the light traveling in a direction oblique to an optical axis of the light source 12 in the light introduction part 15 is reflected by the inclined surface 17 and spread in a horizontal direction (a width direction of the light guide plate) by an arrow in FIG. 1, and an angle formed by the light and the optical axis of the light source 12 increases when the light guide plate body 14 is viewed from above. As a result, the light laterally spread by the inclined surface 17 leaks from a side surface of the light guide plate 13, or the light is laterally incident to the lenticular lens 16 to leak from the lenticular lens 16, whereby the degradation of the light use efficiency or the degradation of luminance evenness is generated by a light quantity loss.
FIG. 2 is a perspective view of the surface light source device disclosed in WO 2010/070821. In a surface light source device 21 in FIG. 2, a plurality of V-groove light leakage prevention patterns 22 parallel to one another are provided in the inclined surface 17 of the light introduction part 15. The light leakage prevention pattern 22 reduces the light leakage from the inclined surface 17 to improve the light use efficiency. However, at the same time, the light reflected by the light leakage prevention pattern 22 is easily diffused in the horizontal direction compared with the case where only the inclined surface 17 is provided. Therefore, the horizontal spread of the light increases, and the light leaks easily from the side surface of the light guide plate 13. Additionally, in the case where optical patterns, such as the lenticular lens, are provided in the upper surface or the lower surface of the light guide plate 13, the light easily leaks from the optical pattern. As a result, the degradation of the light use efficiency or the degradation of the luminance evenness is also generated by the light quantity loss